The demand for traffic capacity, coverage and reliability in the wireless communication systems is seemingly ever-increasing. One bottleneck in the traffic capacity is the limited frequency spectrum available for communication purposes, the limitation being both physical—only part of the frequency spectrum is suitable for communication and the information content per frequency and time is limited, and organisational—the useful part of the spectrum is to be used for a number of purposes including: TV and radio broadcast, non-public communication such as aircraft communication and military communication, and the established systems for public wireless communication such as GSM, third-generation networks (3G), Wireless Local Area Networks (WLAN) etc. Recent development in the area of radio transmission techniques for wireless communication systems show promising results in that the traffic capacity can be dramatically increased as well as offering an increased flexibility with regards to simultaneously handling different and fluctuating capacity needs. Several promising techniques are Multiple-Input-Multiple-Output MIMO) see for example A. Goldsmith et al. “Capacity Limits of MIMO Channels”, IEEE Journal on Selected Areas of Comm., VOL. 21, NO. 5, June 2003, and coherent combining based cooperative relaying, see for example Peter Larsson, “Large-Scale Cooperative Relaying Network with Optimal Coherent Combining under Aggregate Relay Power Constraints”, in Proc. Of Future telecom Conference, Beijing, China, 9-10/12 2003. Compared to presently used transmission techniques such as TDMA (as used in GSM) and WCDMA (as used in UMTS), the above exemplified technique represents a much better usage of the available radio frequency spectrum. As an example of the capabilities of, but also the requirement set forth by, the new transmission techniques, the MIMO wireless systems will be briefly described with references to FIG. 1 (prior art). A comprehensive description of the basic principles as well as recent development and areas of research of MIMO is to be found in the above referred article by A. Goldsmith et al.
A radio link in a MIMO system is characterized by that the transmitting end as well as the receiving end is equipped with multiple antenna elements, as illustrated in FIG. 1. The idea behind MIMO is that the signals on the transmit (TX) antennas at one end and the receive (RX) antennas at the other end are “combined” in such a way that the quality (bit-error rate, BER) or the data rate (bits/sec) of the communication for each MIMO user will be improved. Such an advantage can be used to increase both the network's quality of service and the operator's revenues significantly. A core idea in MIMO systems is space-time signal processing in which time (the natural dimension of digital communication data) is complemented with the spatial dimension inherent in the use of multiple spatially distributed antennas. A key feature of MIMO systems is the ability to turn multipath propagation, traditionally regarded as a limiting factor in wireless transmission, into a benefit for the user. MIMO effectively takes advantage of random fading and when available, multipath delay spread, for multiplying transfer rates. Also schemes such as Transmit Diversity scheme with Rich Feedback (TDRF) and coherent combining based cooperative offer a dramatic increase in capacity and/or quality, as described in “Capacity achieving transmitter and receiver pairs for dispersive MISO channels” by K Zangi and L. Krasny, IEEE Trans. Wireless Commun., July 2002 and in “Optimal and Reduced Complexity Receivers for MISO Antenna Systems” by L. Krasny, S. Grant and K. Molnar, Proceeding IEEE Globecom 2003. The prospect of significant improvements in wireless communication performance at no cost of extra spectrum (only hardware and complexity are added) has naturally attracted widespread attention.
The transmitting principles of a multiantenna system will be described with reference to the schematic illustration of FIG. 1. A compressed digital source in the form of a binary data stream 105 is fed to a transmitting block 110 encompassing the functions of error control coding and (possibly joined with) mapping to complex modulation symbols (quaternary phase-shift keying (QPSK), M-QAM, etc.). The latter produces several separate symbol streams which range from independent to partially redundant to fully redundant. Each is then mapped onto one of the multiple TX antennas 115. Mapping may include linear spatial weighting of the antenna elements or linear antenna space-time preceding. After upward frequency conversion, filtering and amplification, the signals are launched into the wireless channel. N TX antennas 115 are used, and the transmitting block 110 may typically comprise means for N simultaneous transmissions. At the receiver, the signals are preferably captured by multiple antennas (M) 120 and demodulation and demapping operations are performed in the receiving block 125 to recover the message. The level of intelligence, complexity, and a priori channel knowledge used in selecting the coding and antenna mapping algorithms will vary a great deal depending on the application. This determines the class and performance of the multiantenna solution that is implemented.
Naturally, the multiantenna systems offer a transmit-receive diversity gain, similar to the existing smart antenna systems, but can also offer a fundamentally new advantage in the exploration of the space-time. This can be seen as the multiantenna systems transmit data over a matrix channel rather than a vector channel. The signal model of this type of multiantenna system can simplified be described as:r=Hs+n  (1)wherein, r is the M×1 the received signal vector, s is the N×1 transmitted signal vector and n is an vector of additive noise terms, e.g. white Gaussian noise, and H is the M×N channel matrix for the transmitted signals between the transmitter and the receiver.
The multiplexing alone is as previously mentioned, not enough for achieving the dramatic increase in gain. Advanced coding/decoding and mapping schemes, i.e. the space-time coding, is essential. A knowledge of the radio channel is needed for the decoding already in today's existing wireless systems such as GSM and UMTS, and in the multiantenna systems this knowledge is absolutely critical. In some of the most promising implementation proposals for MIMO, the knowledge of the channel, represented by H, is used not only in the decoding performed in the receiver side, but also in the coding on the transmitting side as described in D. Gesbert et al. “From Theory to practice: An Overview of MIMO Space-Time Coded Wireless Systems”, IEEE Journal on Selected Areas of Comm., VOL. 21, NO. 3, April 2003 and in WIPO publication nr WO 03005606.
The knowledge of the characteristics of the channel matrix H at the transmitter can be used to optimize coding and mapping. Not only MIMO systems exploits precise channel state information (CSI), but also for TDRF and coherent combining based cooperative relaying that inherently uses CSI knowledge for optimizing respective communication performance. A forward channel may typically be characterized either by sounding the channel in the forward direction with some training signal and then receive feedback from the other station informing about the channel characteristics, or by receiving a training signal from the other station and acquire knowledge of transmit power. The first alternative can provide a good estimate of the channel characteristics, but at the same time does the transmission of the characteristics of H take up valuable transmission resources. Therefore, a compromise between the increase in gain and the increase in control signalling over payload signalling is typically considered in for example determining suitable update frequency for the characteristics of H. The latter alternative uses less transmission recourses, but relies on the assumption that the channel is reciprocal, i.e. that amplitude and phase are identical regardless of transmission direction. This is e.g. the case in a TDD channel (time division multiplexing) within the channels coherence time. This is particularly true, and of interest, when multiple antennas are used at a first station and only one (or fewer) antennas are used at the other station, as also the number of trailing sequences can be diminished. This is also of great interest for coherent combining based cooperative relaying, as potentially large number of relays (possibly equipped with only one or with few antennas) are exploited while communicating to a user with only one or a few antennas.